The production of plastics in the United States exceeded 22 billion kilograms in 1986, topped 27 billion kilograms in 1991, and reached 35 billion kilograms in 1997. Nearly one third of these plastics were produced for short-term disposable applications such as packaging. As a result, municipal solid waste may contain 7% plastic by weight or 18% by volume.
Most of these synthetic polymeric materials are not susceptible to biodegradation because microbes generally do not contain the enzymes needed to digest structures not occurring in nature, including most monomers in plastics and chiral monomers with the left-handed or "L" conformation. Indeed, most polymers have traditionally been designed for maximum stability.
Massive environmental and disposal problems are associated with this large scale production of plastic wastes. Landfill space is increasingly scarce, with many cities, particularly in the United States, rapidly exhausting their capacity. Potentially, hundreds of thousands of marine animals are killed annually by the estimated one million tons of plastic debris dumped into the world's oceans each year. In addition, the litter is always an aesthetic, as well as an environmental, problem. Recycling of these plastics is hindered by a limited field of applications for recycled plastics and processing difficulties, including sorting of the various types of plastics.
These problems have spurred the development of short-lived plastics for short-term uses. Biological polymers are, by nature, biodegradable, so a viable approach to degradable plastic technology is to find and develop natural polymers that can be produced and processed in place of synthetic equivalents. The biopolymer poly(3-hydroxybutyrate) (PHB) was discovered in 1925 by Maurice Lemoigne of France. PHB and other members of the larger group of polyhydroxyalkanoates (PHAs) are formed naturally in at least 260 species of bacteria. It is believed that these polymers function as the bacterium's source of energy and carbon during periods of starvation.
PHB is a commercially useful polymer that can be completely biodegraded to carbon dioxide and water. Its properties are similar to those of polypropylene, which represented 11% of US polymer production in 1986. In addition, it is human biocompatible, which makes it a useful material for medical implants.
In the late 1980s, the British company, Imperial Chemical Industries (ICI), began small-scale production of a poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) copolymer synthesized in a glucose-utilizing mutant of the bacterium Ralstonia eutropha (until recently, known as Alcaligenes eutrophus). This biopolymer, available under the trade name BIOPOL, was used in shampoo bottles marketed in Germany by the Wella Corporation. However, the expensive feedstocks needed to support bacterial growth, the difficult product extraction, and the limited scale of production limits the commercial production of PHBV.
Cloning and expression of one or more R. eutropha-derived genes involved in the biosynthesis of polyhydroxybutyrate (namely, phbA, encoding .beta.-ketothiolase, phbB encoding NADPH-dependent acetoacetyl-CoA reductase, and phbC encoding PHB polymerase) in E. coli have been described in several United States patents, including U.S. Pat. Nos. 5,245,023; 5,250,430; and 5,534,432 (all to Peoples et al.).
Genetic transformations of plant cells with genes encoding enzymes involved in the biosynthetic pathway for PHB have been reported. For example, Somerville et al. (U.S. Pat. No. 5,650,555) describe an Agrobacterium tumefaciens-mediated transformation of Arabidopsis thaliana with R. eutropha-derived nucleic acid sequences. See also Y. Poirer et al., Science, 256, 520-522 (1992). Expression of the three R. eutropha genes targeted to plant plastids was described by Nawrath et al., Proc. Natl. Acad. Sci. USA, 91, 12760-12764 (1994). Further, John (U.S. Pat. No. 5,602,321) describes fiber-producing cotton plants which have been transformed with a genetic construct that includes a fiber-specific promoter isolated from cotton plants, and a coding sequence selected from the group of sequences encoding ketothiolase, acetoacetyl-CoA reductase, and PHB polymerase.